The present invention relates to a method for determining the actual density of liquids based on the period of a flexural oscillator filled with a fluid, e.g. liquid sample.
It is known to use flexural oscillators for determining the density of liquids and gases on the basis of electronically measuring the period of vibration of the flexural oscillator filled with the liquid from which the density of the liquid sample can be calculated.
To this end, the sample is placed in a hollow, U-shaped tube that forms the flexural oscillator, which is electronically excited to subject it to undamped vibration. The natural frequency of the flexural oscillator depends on the mass of the sample.
The two legs of the U-shaped oscillator tube form spring elements of the flexural oscillator. The direction of vibration is normal to a plane between the two legs.
The natural frequency of the flexural oscillator is affected by only that part of the liquid sample which actually participates in the vibration. The liquid volume that participates in the vibration is limited, for example, by stationary vibration nodes located at clamping points for the flexural oscillator of a natural frequency excitation device. If the oscillator is filled with the liquid sample at least up to the clamping points, the same, precisely defined volume always participates in the vibration, is constant for a given flexural oscillator, and the mass of the sample can be assumed to be directly proportional to its density and vice versa.
Overfilling of the flexural oscillator beyond the clamping points and/or excitation nodes points is irrelevant for the measurement. For this reason, the densities of liquid media flowing through a flexural oscillator can also be continuously measured. The technique of instantaneous direct density measurement of liquid media flowing through a flexural oscillator is used routinely in the beverage, petroleum and chemical industries, for example, and has the essential advantage that density measurements can be performed continuously on flowing liquid media in a fully automated manner.
In addition to temperature, the viscosity of the sample and, unfortunately, also inhomogeneities, e.g. agglomerates, air and/or gas inclusions or the like, if present, can affect the density measurement and falsify the results.
AT 400767B and EP 0568521 A1 address error corrections for such measurements based on the viscosity of the samples. In the disclosed embodiment, a fundamental vibration and a harmonic of same are excited at two different points of a flexural oscillator with an amplifying exciter. The influence of the viscosity of the sample on the measured density value is compensated for as described in detail in the references.
These methods of viscosity correction are also used in the devices currently being made available by applicants, in which excitation of the fundamental vibration and the harmonic in glass flexural oscillators differ somewhat as compared to the method disclosed in AT 400767B. However, this modified type of excitation represents the prevailing state of the art because it is part of the flexural oscillator devices currently used for measuring the density of liquid samples.